Objects emit infrared radiation according to their temperature. An object at room temperature (i.e., 300.degree. K.), for example, emits infrared radiation that has a peak at around 8.5 .mu.m. Even in complete darkness, i.e., in the absence of visible optical wavelengths, the infrared radiation emitted from the object can be detected. That detected radiation can be processed with an infrared-radiation detector to generate an image.
Infrared radiation detectors operating in the range of 8-15 .mu.m have been used in night vision, navigation, flight control, weather monitoring, security, surveillance, and chemical detection. The earth's atmosphere is transparent to 8-12 .mu.m radiation, and infrared-radiation detectors operating in this range are thus used in telescopes, communication systems, and in defense. IR scanner data has also been used to map sulfur dioxide fumes from quiescent volcanos.
The early IR detectors were intrinsic detectors. An intrinsic photodetector takes advantage of optical radiation's capability of exciting a photocarrier, e.g., an electron. Such a photo-excited electron or "photoelectron" is promoted across the band gap from the valence band to the conduction band and collected. The collection of these photoelectrons produces a flow of electrons, which is detected as a current.
An intrinsic photodetector requires that an incoming photon from the radiation to be sensed is sufficiently energetic to promote an electron from the valence band to the conduction band. Hence, the energy of the photon =h.nu. needs to be higher than the band gap E.sub.g of the photosensitive material.
Quantum well detectors are more sensitive. Quantum well photodetectors can be used to form quantum well infrared photodetectors ("QWIPs")that are sensitive to 6-25 .mu.m infrared radiation. A quantum well is formed by packaging a relatively thin layer of a first semiconductor (typically GaAs) between adjacent layers of a second semiconductor (typically Al.sub.x Ga.sub.1-x As). These semiconductor materials have a gap of inherent energies, "a band gap", between them. The materials are used to form an energy "well" in the semiconductor. That well can capture photons generated by the incoming radiation. The electrons are promoted by the photon from a ground state within the well to an excited state.
Spectral response of the detectors has been adjusted by controlling the band gap. However, detection of long wavelength radiation, such as infrared radiation, requires a small band gap; e.g., around 62 meV. These low band gap materials are characterized by weak bonding and low melting points.
The art responded by forming multi-quantum well structures (MQW) made of large band gap semiconductors. Positions of the energy levels in an MQW structure are primarily determined by the well height and width. For example, the energy level separation and the depth of the quantum well are increased as the thickness of the GaAs layer is decreased. The well's height also depends on the band gap of the Al.sub.x Ga.sub.1-x As layer and the relative proportions of Al and Ga ("x") in the Al.sub.x Ga.sub.1-x As. The intersubband energy, i.e., the energy between the ground state E.sub.1 and the first excited state, defines many of the essential characteristics of the quantum well.
Quantum well infrared photodetectors operate based on photoexcitation of an electron between ground and a first excited state in the quantum well. The basic operation of a single well is shown in FIG. 1.
The band gap 110 of the Al.sub.x Ga.sub.1-x As 112 is different from the band gap 120 between the GaAs layers 122. This difference forms the well which captures the electrons. These photoelectrons can escape from the well and are collected as photocurrent.
The band gap of Al.sub.x Ga.sub.1-x As can be changed by varying x. This hence changes the height of the well and allows changing the energy required to capture an electron, the "intersubband transition energy."
An intrinsic infrared photodetector, as described above, increases the energy of an electron using one (or many) photons, and detects the resultant photoelectrons. The photon needs to be sufficiently energetic to increase the energy of the electron sufficiently to promote the electron from the valence band 130 to the conduction band 132. This has been called interband operation, signifying the electron's promotion from one band to another band.
The intersubband system shown in FIG. 1 promotes the electrons between subbands--here from one subband 101 to another subband 106. Intersubband transitions operate between confined energy states, i.e., quantum wells associated with either the conduction band 132 or valence band 130 in the quantum well. The promotion is effective at holes 100 in the quantum well.
Different kinds of intersubband transitions exist. A bound-to-bound transition is formed when both the ground state 104, and the excited state 106 of the excited electrons are bound within a quantum well 100.
A multi-quantum well system is schematically shown in FIG. 2. Like the FIG. 1 system, the quantum wells generate photocurrent following intersubband absorption between two bound energy levels. A bound-to-bound intersubband absorption requires the infrared wavelengths to excite an electron from the ground state 220a to a bound excited state 222 within the well. The electron then tunnels through the edge of the well via quantum tunneling shown as 230, to an unbound and continuous level above the well level, "the continuum level" 210. The bias on the well excites a flow of electrons through the continuum. This flow of electrons is detected as photocurrent.
The sensitivity of the detector is a function of efficiency of the photocurrent detection, i.e., the amount of detected photocurrent sensitivity is degraded by noise in the detector. Since infrared radiation has less energy than higher frequency electromagnetic radiation such as visible electromagnetic radiation, the system generates relatively less photocurrent. This has provided a unique challenge to enhancing detector efficiency.
Dark current is a source of noise in QWIPs. Dark current is, as the name implies, current that flows in the dark, i.e., even when radiation to be detected is not reaching the QWIP. The dark current in a QWIP originates from three main mechanisms, quantum mechanical tunneling, thermally assisted tunneling and thermionic emissions.
Quantum mechanical tunneling from well to well through the barriers (shown as 224), also called sequential tunneling, occurs independent of temperature. This occurs to a very small extent, and dominates the dark current at very low temperatures.
Thermally-assisted tunneling 226 is based on thermally excited quantum tunneling through the tip of the barrier into the continuum 210. At medium temperatures, e.g., around 45.degree. K. for an 8-9 .mu.m detector, thermally-assisted tunneling governs the dark current.
At the more usual high temperatures, greater than 45.degree. K., classical thermionic emissions 228 dominate the dark current. A thermionic emission occurs when the electrons are promoted by thermionic processes, i.e. without an incoming photon.
It is highly desirable to reduce the dark current to make a more sensitive detector, i.e., a detector with higher signal to noise ratio. However, it is also desirable that the detector produce as much photocurrent as possible.
The bound-to-bound system requires a photoexcitation energy, E.sub.P 240 in order to excite it from one state to another. This energy E.sub.P is less than the energy for thermionic emission E.sub.P 242. Since the bound level E.sub.P is within the quantum well, thermionic emission is only caused by those electrons which are sufficiently energetic to escape from that bound level to the continuum 210. The dark current contribution from E.sub.D is hence relatively small.
However, since the excited bound level is within the quantum well, the photoexcited electrons escape from the well by quantum mechanical tunneling shown as 230. The resistance against particle tunneling is inversely and exponentially proportional to the distance through which a particle needs to tunnel. The number of particles which will tunnel through a barrier is inversely exponentially proportional to the thickness of that barrier. Most particles will easily tunnel through a barrier that is less than 50 .ANG. in thickness. However, only some particles will tunnel through a barrier between 50 and 100 .ANG., and any barrier greater than 100 .ANG. in thickness presents a formidable challenge for tunneling. The tunneling for a bound-to-bound transition has typically more than 100 .ANG., and hence many of the electrons do not tunnel in this way. Therefore, while the dark current in the bound-to-bound photodetectors is low, the photocurrent has also been low because of the.
Signal to noise ratio in these detectors can be modeled as: ##EQU1##
Where I.sub.D is the dark current. Both the dark current I.sub.D and the photocurrent I.sub.P are lowered in the bound to bound system.
The level of the bound particles in QWIPs are dependent on characteristics of the QWIP materials. One prior art attempt to increase signal to noise ratio involved reducing the thickness of the GaAs layer in the FIG. 1 system to thereby elevate the excited state energy level into the continuum level. This intersubband configuration has been called "bound-to-continuum." The photoelectrons are bound into the continuum level, so the photoexcited electrons can escape from the quantum well to the continuum transport without tunneling as shown by 254 in FIG. 2. Hence more of the photoelectrons can escape as photocurrent, increasing the signal S. However, since the E.sub.P 250 for this detector is less than the E.sub.D 252, the electrons are very energetic. This configuration hence has a very low barrier against dark current through thermionic emission. The energy barrier for thermionic emission (E.sub.T) is ten to fifteen meV smaller than the energy required for the intersubband photoionization process. Accordingly, this configuration has higher noise N relative to the bound-to-bound system.
A special form of intersubband absorption is described in this specification which increases the signal S while avoiding or minimizing increase in noise. An absorption subband is described which occurs when the first excited state is in resonance with an area near the top of the barrier. The inventors have titled this a "bound-to-quasibound" transition. Such transitions exist when the thermionic emission energy barrier of the quantum well (E.sub.T) is substantially matched to the energy required for photoionization (E.sub.P), i.e., preferably within 2% of precise resonance.
This bound-to-quasibound configuration has a thermionic emission energy barrier which is increased relative to the bound-to-continuum transitions. More thermal energy is required to liberate an electron confined in the quantum well. Dark current generated by the quantum well during operation is therefore reduced. However, since the excited state in the bound-to-quasibound configuration is resonant with the thermionic emission energy barrier, electrons can escape with little or no tunneling. The quantum wells with this configuration hence maintain a high quantum efficiency, i.e., a large amount of photocurrent is generated by the incident infrared photons.
These two factors--low dark current and high quantum efficiency--increase the signal-to-noise ratio of the photocurrent generated by the quantum well.
It is hence an object to increase the energy barrier for thermionic emission relative to bound-to-continuum structures. One aspect of the present invention carries out this object by forming bound-to-quasibound quantum wells which exhibit increased sensitivity and improved dynamic range.
The quantum's depth and thickness of the quantum well are modified so that the first excited state is resonant with (i.e., has substantially the same energy as) a portion of the "bottom" (i.e., the lower energy barrier) of the quantum well. The energy barrier for thermionic emission is thus substantially equal to the energy required for intersubband absorption. Increasing the energy barrier in this way significantly reduces dark current while the photocurrent generated by the quantum well is maintained at a high level.
Bound-to-quasibound QWIPs exhibit peak sensitivities at a value that is based on the material thicknesses. An exemplary value is 8.5 .mu.m at 70.degree. K. However, this value can be changed by appropriate adjustment of the well width L.sub.W and the barrier height E.sub.G.
A single QWIP includes a quantum well structure with about 50 quantum wells. Each well preferably includes a 40-70 .ANG. thick GaAs between two 300 .ANG.-500 .ANG. Al.sub.x Ga.sub.1-x As barrier layers. The mole fraction (x) of Al is preferably 0.3. Each quantum well is preferably doped with a density of n-type carriers (typically 5.times.10.sup.17 cm.sup.-3) to lower the Fermi energy of the carriers and further reduce dark current.
The QWIP quantum well structure of the preferred embodiment is formed between silicon-doped GaAs electrical contact layers. These layers are attached to electrical leads which supply an electrical bias which facilitates collection of photocurrent. All layers are preferably formed on a GaAs substrate.
The QWIP primarily absorbs radiation having a polarization component along the growth axis (i.e., thickness) of the quantum well. The QWIP thus preferably includes a randomly reflecting surface (e.g., gold) patterned on the top electrical contact layer. Radiation passes into and through the bottom of the QWIP and irradiates the reflecting surface. Internal reflections within the quantum well structure adjust angles of the radiation relative to the growth axis to facilitate optical absorption. The number of internal reflections is maximized by making the GaAs substrate as thin as possible.
QWIPs accordingly to the embodiment are preferably patterned on the GaAs substrate in a 256.times.256 pixel array although other sizes are contemplated. This structure is preferably incorporated in a QWIP/silicon CMOS multiplexer hybrid detector used for generating two-dimensional infrared images.
QWIP pixel arrays are formed by first growing a stop-etch layer and an electrical contact layer on a 3-inch GaAs wafer. This area is called the "bottom" of the structure. The quantum well structure is formed by growing alternating Al.sub.x Ga.sub.1-x As barrier and GaAs well layers on top of the electrical contact layer. A final electrical contact layer is then grown on top of the quantum well structure.
Each layer is grown by molecular beam epitaxy (MBE). Multiple QWIP arrays are then patterned using standard photolithography and chemical etching techniques. The GaAs wafer is then diced to form individual GaAs substrates, each containing a single focal plane array.
The system of the present invention is often used in special cameras and systems. The focal plane array described in this specification is often used as a hybrid along with its CMOS support circuitry. This produces its own special host of problems.
A single GaAs focal plane array is attached to the CMOS multiplexer pixel array and "thinned" substantially down to the stop-etch layer. A special photolithographic thinning process which reduces the aspect ratio (i.e., the ratio of thickness to width) of the QWIP array is described. Thinning the substrate improves optical coupling, minimizes thermal mismatch between GaAs and CMOS logic families, and minimizes optical crosstalk between adjacent pixels.
The thinning process includes an abrasive polishing step for removing the first 500 .mu.m of the substrate. A chemical polishing step (using a bromine:methanol mixture at a ratio of 1:100) is then used to remove the next 100 .mu.m of substrate. Outer surfaces of the QWIP photodetector except for the GaAs substrate surface are then covered with a standard photoresist. A wet chemical etching step using a H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 :H.sub.2 O solution (5:40:100) removes the next 20 .mu.m from the substrate. The etching process is continued until about 5 .mu.m of the GaAs substrate remains. The detector is then loaded into a plasma etching chamber evacuated to a pressure of less than 1.times.10.sup.-6 torr. CCl.sub.2 F.sub.2 flows in the presence of a radio frequency ("RF") to form a plasma in the chamber to etch the substrate until the stop-etch layer is reached. The thinned QWIP pixel array attached to the CMOS multiplexer pixel array is then processed with a final cleaning step and removed from the chamber.
Bound-to-quasibound QWIPs exhibit relatively low amounts of dark current due to their increased energy barriers for thermionic emission. Lowering the dark current increases the sensitivity and dynamic range of the QWIP. The first excited state of the bound-to-quasibound QWIP is resonant with the top of the well. This configuration maintains the quantum efficiency, of the QWIPs (i.e., the number of photocarriers generated for each incident photon) and sensitivity at a high level.
QWIPs according to the techniques described in this specification can be used to form a photodetector having high-quality images, having high signal-to-noise ratios.